1. Technical Field
The present invention relates generally to medical diagnostic imaging and to use of medical images in one imaging modality with medical images from another imaging modality. More specifically the invention relates to extension or extrapolation of image data from a Computed Tomography (CT) transmission scan having a limited Field of View (FOV) so as to extend the CT image to a larger FOV. The FOV extension increases the anatomic information provided by the CT scan, and it makes the attenuation data provided by the CT image more accurate for correcting radionuclide emission data obtained from an emission computed tomography scan such as Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT). In particular, the invention relates to the use of a process, which could be iterative, for reconstruction of a CT image from transmission image data. This process compensates for missing transmission data from an extended FOV outside the CT measurement field. This extended FOV could be associated with that part of the FOV of a corresponding PET or SPECT scan of the same region that lies outside the CT FOV. The extended CT data can be used, for example, to generate attenuation correction factors for the corresponding PET or SPECT data, as well for reconstruction of CT images for co-registration with PET or SPECT reconstructed images.
2. Description of the Related Art
Single Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET), and Computed Axial Tomography (CT) are three medical imaging modalities. SPECT, PET, and CT are popular in medicine because of their ability to non-invasively study both physiological processes and structures within the body. To better utilize SPECT, PET and CT, recent efforts have been made to combine either a SPECT scanner with a CT scanner or a PET scanner with a CT scanner into a single system. The combination of SPECT and CT or of PET and CT allows for better registration of the metabolic or functional SPECT and PET images with the anatomic CT image and for improved hospital workflow. The combined scanners share space within the same system housing and share a common patient bed or gurney, but use separate detectors and associated hardware. In the case of the PET/CT scanner see, for example, U.S. Pat. No. 6,449,331, issued to Nutt, et al., on Sep. 10, 2002, entitled “Combined PET and CT Detector and Method for Using Same,” which discloses a combined PET and CT scanner, and which is incorporated herein by reference in its entirety.
SPECT and PET are nuclear medicine imaging techniques used in the medical field to assist in the diagnosis of diseases. In both cases medical images are regenerated based on radioactive emission data, typically in the form of gamma rays, emitted from the body of a patient after the patient has ingested or been injected with a radiopharmaceutical substance. SPECT and PET allow the physician to examine large sections of the patient at once and produce pictures of many functions of the human body unobtainable by other imaging techniques. In this regard, SPECT and PET display images of how the body works (physiology or function) instead of simply how it looks (anatomy or structure).
Mechanically, a SPECT or PET scanner consists of a bed or gurney and a gantry, which is typically mounted inside an enclosure with a tunnel through the center, through which the bed traverses. The patient, who has been infused with a radiopharmaceutical, lies on the bed, which is then inserted into the tunnel formed by the gantry. The gantry is rotated (either physically or electronically simulated with a stationary ring) around the patient as the patient passes through the tunnel. The rotating gantry contains the detectors and a portion of the processing equipment.
In the case of SPECT, emitted gamma rays are detected from numerous different projection angles by a gamma camera (a.k.a. Anger camera or scintillation camera) about a longitudinal axis of the patient, and converted into electrical signals that are stored as image data. Data from image projections provide a set of images as a result of a process known as image reconstruction. In the case of PET, the scanner detectors are designed to detect simultaneous and oppositely traveling gamma ray pairs from positron annihilation events within the patient. The injected or ingested radiopharmaceutical contains positron-emitting atoms. The positrons annihilate with electrons in the patient to produce pairs of gamma rays where each member of the pair moves in an opposite direction. The paired gamma rays generate signals when they strike the PET scanner detectors. Signals from the gantry are fed into a computer system where the data is then processed to produce images as a result of a process known as image reconstruction.
PET is considered the more sensitive of the two nuclear medicine imaging techniques, and exhibits the greatest quantification accuracy, of any nuclear medicine imaging instrument available at the present time. Applications requiring this sensitivity and accuracy include those in the fields of oncology, cardiology, and neurology.
Another known tomography system is computed axial tomography (CAT, or now also referred to as CT, XCT, or x-ray CT). In CT, an external x-ray source is caused to be passed around a patient. Detectors on the other side of the patient from the x-ray source then respond to the x-ray transmission through the patient to produce an image of the area of study. Unlike SPECT or PET, which are emission tomography techniques because they rely on detecting radiation emitted from inside the patient, CT is a transmission tomography technique which utilizes a radiation source external to the patient. CT provides images of the internal structures of the body, such as the bones and soft tissues, whereas SPECT and PET provide images of the functional aspects, such as metabolism, of the body, usually corresponding to an internal organ or tissue.
Unlike the pairs of PET scanner detectors required to detect the gamma ray pairs from an annihilation event or the detector heads of the SPECT scanner, the CT scanner requires detectors mounted opposite an x-ray source. In third-generation computed tomography systems, the CT detectors and x-ray source are mounted on diametrically opposite sides of a gantry which is rotated around the patient as the patient traverses the tunnel.
The x-ray source emits a beam of x-rays which pass through the patient and are received by an array of detectors. As the x-rays pass through the patient, they are absorbed or scattered as a function of the densities of objects in their path. The output signal generated by each detector is representative of the x-ray attenuation of all objects between the x-ray source and the detector.
The medical images provided by the SPECT/CT scanner or by the PET/CT scanner are diagnostically complementary, and it is advantageous medically to have images of the same region of a patient from both emission and transmissions scans. To be most useful, the SPECT and CT images or the PET and CT images need to be correctly overlaid or co-registered such that the functional features in the PET images can be correlated with the structural features, such as bones, tumors, and lung tissue, in the CT images. The potential to combine functional and anatomical images is a powerful one, and there has been significant progress in the development of multi-modality image co-registration and alignment techniques. However, with the exception of the brain, the co-alignment of images from different modalities is not straightforward or very accurate, even when surface markers or reference points are used. To this end, it is desirable to incorporate SPECT and CT scanners or PET and CT scanners into a single gantry, thereby allowing the image data to be acquired sequentially or possibly simultaneously within a short period of time on the same patient table and overcoming alignment problems due to patient movement or internal organ movement such as caused by cancer treatment, respiration, variations in scanner bed profile, positioning of the patient for the scan, and other temporal changes in the patient.
SPECT and PET image reconstruction methods include iterative methods, such as ART (Algebraic Reconstruction Technique), EM (Expectation Maximization), ML (Maximum Likelihood), OSEM (Ordered Subset Expectation Maximization), and MAP (Maximum A Posteriori), as well as traditional (non-iterative) reconstruction methods, such as filtered back-projection (FBP). (OSEM is a modified EM technique based on ordered subsets.) Iterative reconstruction methods often provide better image quality and more natural ways to incorporate attenuation correction than non-iterative methods. However, iterative methods are generally more computationally intensive and more time-consuming than non-iterative methods. In fact, iterative techniques can be on the order of ten times slower than non-iterative techniques. Consequently, in the past iterative techniques were not used in CT as sufficient computational power was cost prohibitive for the resolution and contrast requirements of that modality.
Over a period of approximately fifteen years a number of studies have addressed attenuation correction of SPECT or of PET emission images obtained using transmission images reconstructed with tomographic techniques. According to U.S. Pat. No. 5,376,795, photon attenuation constitutes a major deficiency in diagnosis of heart disease with SPECT and is a major source of error in the measurement of tumor metabolism using radionuclide techniques. A number of researchers have shown that transmission imaging techniques can be used to create attenuation maps that can be used to correct photon attenuation in emission image data. These techniques include CT.
An example of the prior art in attenuation correction for emission computed tomography with transmission computed tomography is U.S. Pat. No. 6,339,652, titled, “Source-assisted attenuation correction for emission computed tomography.” This patent describes a method of ML-EM image reconstruction that is provided for use in connection with a diagnostic imaging apparatus that generates projection data. The method includes collecting projection data, including measured emission projection data and measured transmission projection data. Optionally, the measured transmission projection data is truncated. An initial emission map and attenuation map are assumed. The emission map and the attenuation map are iteratively updated. With each iteration the emission map is recalculated by taking a previous emission map and adjusting it based upon: (i) the measured emission projection data; (ii) a re-projection of the previous emission map which is carried out with a multi-dimensional projection model; and, (iii) a re-projection of the attenuation map. As well, with each iteration the attenuation map is re-calculated by taking a previous attenuation map and adjusting it based upon: (i) the measured emission projection data; (ii) a re-projection of the previous emission map which is carried out with the multi-dimensional projection model; and (iii) measured transmission projection data.
Other studies in this area include disclosures of U.S. Pat. No. 5,739,539, which describes a method of performing image reconstruction in a gamma camera (SPECT) system that includes the steps of performing a transmission scan of an object about a number of rotation angles to collect transmission projection data and performing an emission scan of the object about numerous rotation angles to collect emission projection data. The outer boundary of the object is then located based on the transmission projection data. Information identifying the boundary is then either stored in a separate body contour map or embedded in an attenuation map. Information identifying the boundary can be in the form of flags indicating whether individual pixels are inside or outside the boundary of the object. The emission projection data is then reconstructed using the attenuation map, if desired, to generate transverse slice images. Image reconstruction requires less time if the process ignores pixels outside the body boundary.
U.S. Pat. No. 6,856,666 describes multi-modality imaging methods and apparatus for scanning an object in a first modality, having a first field of view to obtain first modality data including fully sampled field of view data and partially sampled field of view data. The method also includes scanning the object in a second modality having a second field of view larger than the first field of view to obtain second modality data, and reconstructing an image of the object using the second modality data and the first modality partially sampled field of view data.
U.S. Pat. No. 6,140,649 titled “Imaging attenuation correction employing simultaneous transmission/emission scanning,” describes a nuclear medical imaging system generates transmission and emission images simultaneously. The system includes a gamma camera and a linear transmission source disposed on opposite sides of an imaging region in which a patient lies. A plurality of views is taken at different rotational angles around a patient. At each angle, the view acquisition period is divided into two segments based on whether the transmission source is on or off. Emission image data is acquired either in both period segments or only while the transmission source is off. The transmission image data is acquired when the transmission source is on, and crosstalk image data is acquired when the transmission source is off.
U.S. Pat. No. 5,338,936 titled “Simultaneous transmission and emission converging tomography,” describes a SPECT system that includes three gamma camera heads which are mounted to a gantry for rotation about a subject. The subject is injected with a source of emission radiation, which emission radiation is received by the camera heads. Transmission radiation from a transmission radiation source is truncated to pass through a central portion of the subject but not peripheral portions and is received by one of the camera heads concurrently with the emission data. As the heads and radiation source rotate, the transmitted radiation passes through different parts or none of the peripheral portions at different angular orientations. An ultrasonic range arranger measures an actual transmission data using an iterative approximation technique and the measured actual subject periphery. The actual periphery is used in the reconstruction process to reduce artifacts attributable to radiation truncation and the associated incomplete sampling of the peripheral portions. An emission reconstruction processor reconstructs the emission projection data and attenuation properties into an attenuation corrected distribution of emission radiation sources in the subject.
Each improvement in co-registration of multimodality image data and attenuation correction of nuclear medicine images provides benefits associated with the quality of medical diagnoses. For this reason there is continuing need for methods of image reconstruction for reliable reproduction of a patient's physical and functional condition.
One limitation of attenuation maps created by CT transmission data is the relatively small usable FOV (typically on the order of 50 cm diameter) compared to the FOV of a PET scanner (typically on the order of 70 cm diameter or more). Often the patient's arms are truncated in the CT image because they extend beyond the CT FOV. Also, patients positioned off-center on the bed or obese patients may have a portion of their body truncated for the same reason. Various methods have been developed to reconstruct and extend the FOV of CT images. However, for various reasons, these attempts either do not extend the FOV far enough or introduce artifacts and/or distortion in the images.
Current methods for extending the FOV of CT consist of extrapolating the projection data into the truncated region and reconstructing the image from the extrapolated data by variants of the convolution/back projection (or filtered back projection) reconstruction technique. Prior art methods for reconstructing CT images to extend the FOV are known from the literature. However, these prior art methods have proven unacceptable for reconstructing truncated views.
A representative example of the prior art is an article titled “A novel reconstruction algorithm to extend the CT scan field-of-view,” authored by J. Hsieh, et al., published in the journal Med. Phys. 31(9) September, 2004. This article discloses a reconstruction algorithm that makes use of the fact that the total attenuation of each ideal projection in a parallel sampling geometry remains constant over views. The magnitudes and slopes of the projection samples at the location of truncation are used to estimate the projection data outside the scan field of view.
Another example of the prior art is an article titled “Efficient correction for CT image artifacts caused by objects extending outside the scan field of view,” authored by B. Ohnesorge, et al., published in the journal Med. Phys. 27(1), January, 2000. This article discloses a method of eliminating CT image artifacts generated by objects extending outside the scan FOV. The abstract states: “CT projection data are measured only within the scan field of view and thus are abruptly discontinuous at the projection boundaries if the scanned object extends outside the scan FOV. This data discontinuity causes an artifact that consists of a bright peripheral band that obscures objects near the boundary of the scan FOV. An adaptive mathematical extrapolation scheme with low computational expense was applied to reduce the data discontinuity prior to convolution in a filtered back projection reconstruction.”